I have a Windows7 64-bit machine attached to a 27" iMac as an external displayport monitor It's working well, except I cannot adjust the brightness.

I have attached a Mac keyboard both on the Mac & Windows side to gain access to the brightness controls, with no luck. As soon as I switch to display target mode, the screen jumps to full brightness, and cannot be controlled from the keyboard on either platform.

3 Answers
3

I've futzed with this for a while using a MBP and an iMac, both
running 10.6.8. The problem is inconsistent for me, but seems to be
linked to how long I've had the iMac on. If on chronically (as I
usually leave it), the iMac tends not to respond to F1/F2 brightness
keystrokes when I connect my MBP and use the iMac in target display
mode. But if I reboot the iMac and then connect from my already
running MBP, this restores brightness keystroke response. That is:

It is probably easier to adjust the brightness in windows from the display drivers settings. usually you can right click empty desktop and the display drivers UI is the first entry in context menu (like "AMD Vision Control Center"). All three major video card vendors can adjust brightness on the drivers side.

It is confirmed that should be able to control that with iMac's keyboard. I've tried but it doesn't work in your setup.

Also, the brightness doesn't show up under OS X's System Preferences, and I have no chance to test it under Windows since my Thunderbolt Macs will only connect under OS X, not under Windows 8 Boot Camp.